1. Field of the Invention
The invention concerns a paper for smoking articles having low ignition propensity.
2. Description of Related Art
Conventionally, cigarette papers intended for the production of industrial cigarettes are made from cellulose fibres (fibres from wood and/or plant textile fibres with the addition of calcium carbonate to the fibrous suspension as conventional pigment).
Combustion delaying or accelerating salts are conventionally applied over the entire surface during manufacture, to gain control over some burn parameters of the formed cigarette. These are generally sodium salts, potassium salts, magnesium salts, etc. They also impart improved combustibility to cigarettes.
Current standards require cigarette manufacturers to observe levels of tar, nicotine, carbon monoxide (CO) per cigarette lying below given thresholds. For example, European regulations require thresholds of 10 mg per cigarette for tar, 1 mg per cigarette for nicotine and 10 mg per cigarette for carbon monoxide.
It has been ascertained that the reduction in condensates of the particle phase (tar and nicotine) and of carbon monoxide in cigarette smoke is proportional to the increase in natural porosity of the paper. For example, the use of paper having high initial permeability of between 10 and 200 Coresta (CU, or mL/min/cm2) allows a reduction to be obtained of 28% for tar, about 20% for nicotine and 45% carbon monoxide.
The most part of this gain is acquired as soon as the level of 70 Coresta is reached, with an additional reduction over the range 100-200 CU.
Paper manufacturers, moreover, have been led to proposing papers having low ignition propensity to limit the risks of self-combustion of cigarettes. The objective of these papers is to achieve extinguishing of the cigarette if combustion is not maintained through a supply of oxygen i.e. if the smoker does not “draw” on the cigarette. These papers are currently known as “LIP” papers (for Low Ignition Propensity) and comprise LIP-bands treated with a film-forming formulation adapted to block the pores of the paper and thereby reduce the permeability of the paper in these areas. The alternation of areas treated with film-forming formulation and of non-treated areas allows the ignition propensity of the paper to be reduced by partially depriving of oxygen the burning cone of the cigarette when it reaches the areas of low permeability (closed).
However the LIP areas have a harmful effect on tar, nicotine and carbon monoxide levels per cigarette, since they reduce the natural porosity of the paper. It has therefore been proposed to increase initial porosity significantly by applying combustion salts to the paper before treating some areas with the film-forming formulation.
It has also been proposed to coat all or part of the paper with burn delaying salts which cause endothermic reactions during combustion of the paper. Their combustion, on the other hand, generates carbon dioxide (CO2), dinitrogen (N2) and water.
The treated areas are generally transverse rings formed on all or part of the cigarette. Nevertheless, the discrete treatment of the sheet of paper in successive bands, separated by areas not treated with the film-forming formulation, sets up stresses in the sheet of paper which often generate problems when processing the paper, in particular when the treated paper is spooled. The paper effectively has a tendency to bulge outwardly at the localized areas.
Here the propensity of cigarettes to cause fire was evaluated following the ASTM E 2187-04 test method. This test method measures the probability that a cigarette placed on a substrate produces sufficient heat to maintain burning of the tobacco column irrespective of the composition of the tobacco used. Each determination consists of placing a lit cigarette on a horizontal surface formed of a given number of layers of filter paper (ten thicknesses).
It is then determined whether the cigarette continues to burn its full length as far as the end-tip paper.
Forty determinations (forming one test) are conducted to obtain the relative probability that the cigarette will continue to burn despite absorption of heat by the substrate.
In addition to the evaluation test for ignition propensity as per the ASTM E 2187-04 test method, it is also possible to evaluate the percentage number of cigarettes which self-extinguish in free air (EASE test, for Free Air Self Extinguishment). Free combustion is characterized here by the capacity of the lighted cone of the cigarette to travel along the full length of the cigarette despite the presence of treated areas, without any drawing on the cigarette.
Finally, a diffusivity test was also carried out, allowing more rapid and easier prediction of the LIP nature of a paper. This test was conducted on LIP-treated areas by measuring the capacity of the paper to diffuse carbon dioxide. Prediction gives good results when the diffusivity of carbon dioxide is lower than 0.3 cm/s, and more preferably lower than 0.2 cm/s.
The apparatus used to measure diffusivity was SODIM D-95 diffusion measurement equipment.
The formulations containing film-forming compounds are generally applied by printing, typically by heliograph, serigraphy or flexography, and they must therefore have particular dry extract and viscosity characteristics.
It has been observed however that the use of LIP papers affects the functional aspects of a cigarette, in particular the taste, ash integrity, effective carbon monoxide level, etc. Also, it has been ascertained that when a smoker re-lights a cigarette at one of the LIP-treated areas, the taste and carbon monoxide level are modified.